1. Field of the Invention
The present invention relates to the preparation of zeolites of type ZSM-5.
2. Description of the Prior Art
The zeolites of type ZSM-5 are well known materials which have, in particular, been described in U.S. Pat. No. 3,702,886 and British Patent Specification No. 1,161,974.
These zeolites of type ZSM-5 have been manufactured by a variety of processes. Thus, there has already been proposed, in French Patent No. 1,587,860, a process for the synthesis of ZSM-5 by mixing a source of aluminum, a source of silicon, an inorganic base, namely sodium hydroxide, and an organic compound selected from among compounds containing the tetrapropylammonium ion, the amines or the aminoalcohols, after which crystallization is carried out at a temperature of about 150.degree. C. The principal disadvantages of this process reside firstly in the very long time required for the crystallization and secondly in the use of very expensive organic compounds.
It has also been proposed, for example in U.S. Pat. No. 4,257,885 and in British Patent Specification No. 1,567,948 to carry out the synthesis of a zeolite of type ZSM-5 without using organic compounds containing an ammonium cation. In U.S. Pat. No. 4,257,885, seeds of the zeolite desired to be manufactured are employed, and the reaction is carried out in an aqueous medium. According to British Patent Specification No. 1,567,948, it is also possible to proceed without the seeds, but in that case the synthesis is carried out in an aqueous-alcoholic medium. These processes however suffer from the disadvantage of being lengthy to carry out because of the very slow kinetics of the reactions.